1. Field of the Invention
The present invention relates to an improved ozone generating apparatus.
2. Description of the Prior Art
It has been known that ozone can be generated by a discharge and can be used in various fields as an oxidizing agent or a bactericide. Recently, the applications of ozone have increased, especially in the field of elimination of pollution, ozone being used for treatment of sewage, industrial drainage and nitrogen oxides NO.sub.x in effluent gas.
FIG. 1(a) is a schematic view of a conventional ozone generating apparatus wherein a glass discharge tube 13 is disposed in the center of a metallic cylinder 12 and a metallic electrode 14 is adhered or vapormetallized on an inner surface of the glass discharge tube 13 and is connected to a high voltage terminal 15.
A power source 1 is connected to a high voltage terminal 15 and the metallic cylinder 12 on its outer surface so as to apply a sinusoidal waveform voltage having commercial or high frequency. Oxygen in the air fed from one end of the metallic cylinder 12 is converted to ozone by the discharge in the gap between the metallic cylinder 12 and the glass discharge tube 13.
FIG. 2 is a circuit diagram of the power source system of the conventional ozone generating apparatus.
FIGS. 3(a), 3(b) are schematic operating waveforms of power voltage V and current i of the ozone generator.
In FIG. 2, the reference 1 designates the power source for driving the ozone generator (such as a power source having commercial frequency); 2 designates a boosting transformer; 3 designates an ozone generator and 4 designates a reactor for power-factor improvement. When the power voltage is the sinusoidal waveform voltage V.sub.(t) of FIG. 3(a), the ozone generator 3 is an equivalent capacitor whereby the phase gains .pi./2 (rad) from the power voltage as shown in FIG. 3(b). Accordingly, the current waveform is significantly changed during the discharge period T.sub.E and the current i.sub.(t) as a part of the sinusoidal waveform is fed during the non-discharge period T.sub.D.
The ozone generator can be considered equivalent to the series circuit of a capacitor C.sub.g formed by the glass discharge tube 13 and a capacitor C.sub.a formed by a gap between the metallic cylinder 12 and the glass discharge tube 13 as shown in FIG. 1(b). In FIG. 1(b), when the discharge occurs in the gap, the capacitor C.sub.a is considered as a short-circuit and only capacitor C.sub.g remains in the equivalent circuit. This is schematically shown as turning on the switch S.
The operation of the conventional ozone generator will now be illustrated. The ozone generator is considered as a series circuit of the capacitors C.sub.g and C.sub.a wherein C.sub.a &lt;&lt; C.sub.g, in general. As shown in FIG. 1(b), the voltage applied to the ozone generator is shown as V and the partial voltage for the capacitor C.sub.g is shown as V.sub.g and the partial voltage for the capacitor C.sub.a is shown as V.sub.a. When the sinusoidal waveform voltage V is applied to the ozone generator, and as shown in FIG. 4, the terminal voltage V.sub.a of the capacitor C.sub.a reaches the positive discharge voltage V.sub.s at the time t.sub.1 and the discharge in the gap occurs to provide O(V) of V.sub.a. Prior to the discharge, the terminal voltage V.sub.g of the capacitor C.sub.g which is formed by the glass discharge tube is not substanitally changed under the relation of C.sub.a &lt;&lt; C.sub.g as shown by the dotted line.
However, when the discharge occurs as the short-circuit of C.sub.a at the time t.sub.1, all of the power voltage V is applied to C.sub.g to provide V(t.sub.1) = V.sub.g (t.sub.1) at the time t.sub.1 and V.sub.g rises to V(t.sub.1) at the time t.sub.1 as shown by the dotted line. The discharge is finished in a moment and the voltage is again applied to C.sub.a.
The change of the voltage V of the ozone generator appears substantially as the change of the terminal voltage V.sub.a of C.sub.a under the relation of C.sub.a &lt;&lt; C.sub.g and V.sub.g is not substantially changed. Accordingly, the change of V.sub.a is substantially the same as that of V and the discharge occurs at the time t.sub.2 in V.sub.a = V.sub.s. At the time t.sub.2, the voltage V.sub.g becomes V(t.sub.2) = V.sub.g (t.sub.2) whereby V.sub.g rises as shown by the dotted line. The phenomenon is repeated until the time t.sub.5.
After the time t.sub.5, V.sub.a changes substantially the same as V. However, V.sub.a .gtoreq. V.sub.s is not realized until the maximum value of V is reached and V.sub.a falls similar to the change of V until the time t.sub.6. During this period, V.sub.a is changed from positive through zero to negative. At the time t.sub.6, V.sub.a = -V.sub.s is equal to the negative discharge voltage. At the time t.sub.6, the discharge in the negative side occurs to give V.sub.a = 0.
At the time t.sub.5, V.sub.g becomes V.sub.g = V(t.sub.5) and then V.sub.a is kept at a substantially constant value. However, at the time t.sub.6, when the discharge occurs for C.sub.a to give V.sub.a = 0 V.sub.g suddenly falls as shown by the dotted line because V(t.sub.6) = V.sub.g. After the time t.sub.6, the same condition is repeated to give V.sub.a = -V.sub.s at the times t.sub.7, t.sub.8, t.sub.9 and t.sub.10 and the discharges for C.sub.a occur at these times to change V.sub.g as shown by the dotted line.
Accordingly, when the ozone generator is used by applying a sinusoidal waveform voltage, the following conditions are realized.
(1) In one cycle period of the voltage T.sub.o, the discharge phenomenon maintaining period is 2T.sub.E from t.sub.1 to t.sub.5 and from t.sub.6 to t.sub.10 as shown in FIG. 4 and the discharge ceasing period is 2T.sub.D from t.sub.5 to t.sub.6 and from t.sub.10 to t.sub.11. Thus, only about 50% of one cycle period is the discharge phenomenon maintaining period.
(2) The voltage V.sub.a is changed under substantially the same condition as that of V because of a constant of .+-.V.sub.s of the discharge voltage in the gap and the fact that C.sub.g &gt;&gt; C.sub.a. Accordingly, the discharge interval is short around the zero point of the voltage V wherein dv/dt is high and the discharge interval increases and is longest around the maximum value of the voltage V wherein dv/dt is low.
Thus, t.sub.s &lt; t.sub.L as shown in FIG. 4. However, in the conventional ozone generating apparatus, the power P.sub.o is fed during the short period of 2T.sub.E which is realized by subtracting 2T.sub.D from T.sub.o whereby heat is generated during the short period in a concentrated condition.
Accordingly, the yield of ozone is decreased because of the rising temperature of the molecules in the gap and the discharge tube may be damaged because of the thermal and the mechanical stress of the glass discharge tube for the ozone generator.
In the case of operation by applying the conventional sinusoidal waveform voltage, dv/dt of V.sub.t is changed during the operation and the discharge voltage V.sub.s in the gap remains constant. Accordingly, the frequency for repeating the discharges in the initial discharge period near the time t .sub.1 and t .sub.6 is high and the frequency gradually decreases. The power is concentrated near the zero point of the power voltage thereby decreasing the yield of ozone and increasing the thermal and mechanical stress for the glass discharge tube as above-mentioned.